Analyzing samples with dilute components may be challenging because target concentrations may be below detection limits of conventional diagnostic techniques. For example, cardiac troponin I (cTnI) is a low-abundance biomarker useful for diagnosing patients for myocardial injury. In particular, a ratio between phosphorylated and unphosphorylated cTnI may be used as an indicator of patients' risk of suffering myocardial damage. However, normal cTnI levels in healthy people are very low, and thus obtaining baseline cTnI levels is difficult. Conventional techniques for measuring such low levels of cTnI include non-equilibrium isoelectric focusing, mass spectrometry, and phosphate-affinity sodium dodecyl sulfate polyacrylamide gel electrophoresis (“SDS-PAGE”). These conventional techniques, however, are complex, costly, and low in throughput.